Plastic flame spray coatings are generally prepared in the art from powdered plastic applied with a flame spray gun. The flame spray gun typically propels a central stream of pneumatically conveyed finely-divided thermoplastic material through a flame and onto the substrate surface to be coated. The thermoplastic becomes molten from the heat of the flame and is deposited onto a substrate surface where it cools and hardens to form a surface coating.
Flame spray guns are well known in the art. These guns are widely used for the application of metallic, ceramic and metallic-ceramic coatings. Typical of flame spray guns are, for example, the guns described in U.S. Pat. Nos. 4,934,595 and 4,632,309 to Reimer. In these guns, a stream of particulate material entrained in pressurized conveying air, a stream of pressurized combustion and propelling air, and a stream of fuel gas, are delivered in a concentric annular configuration to a combustion chamber such that the particulate material stream passes through a flame tunnel. Special considerations are given for enhancing the diameter and length of the flame tunnel to maximize the rate at which the particulate material can be applied to the substrate surface.
Plasma spray guns are also used, and differ primarily in that the particulated material is heated by passing it through hot plasma gas propelled from the gun in place of the oxy-fuel flame of the flame spray gun. International Publication WO 90-14895 describes an autogenic flame injection apparatus which can be used for either flame spraying or plasma application of powdered metals, ceramics, ceramic-metal mixtures and plastics.
In the present art, a variety of approaches are attempted in efforts to provide suitable plastic coatings made from thermoplastic materials. One approach is to employ plastic materials without inherent chemical functionality that can develop adhesiveness in the flame spray application, such as, for example, low density polyethylene. In this approach, applicators must successfully execute controlled oxidation of either or both the polymer composition and the substrate to gain adequate adhesion. For example, in U.S. Pat. No. 2,718,473 to Powers, anatase (TiO.sub.2) was added to flame sprayed polyethylene powder to obtain controlled oxidation of the polyethylene to enhance adhesion of the resultant polyethylene coating. However, because of the stringently controlled temperature and oxidative conditions necessary to achieve successful coating, the use of polymer compositions without inherent chemical functionality has not been very successful commercially, particularly in field applications where such stringent control is frequently difficult if not impossible.
As another approach, it has been known to employ polymer compositions such as polyether imides, which are not inherently susceptible to thermal oxidation at practical thermal spray application temperatures. These high temperature materials, however, are very expensive and do not always have the inherent performance properties required or desired for specific protective coating and end-uses.
It has also been known to use inherently adhesive polymer compositions, such as those based on ethylene-acrylic acid copolymers. While this has some advantage over the unmodified polyethylene flame spray coating materials and is less expensive than the polyether imides, there remains much room for improvement.
A number of variables interplay in the application of flame spray thermoplastic coatings. For example, melt rheology and adhesion as previously noted, are of primary concern. In the flame spray application of thermoplastics, the substrate surface must generally reach a minimum "wet-out temperature" in order to obtain initial adhesion of the flame spray coating material. A low melt viscosity is generally desirable in order to reduce the wet-out temperature and impart initial adhesion. On the other hand, if the melt viscosity is too low, the molten plastic may, for example, run or ripple before cooling such that there are defects in the resultant coating. Also, lower melt viscosity polyolefins will have lower average molecular weights and concomitantly inferior mechanical properties.
The coating thickness is also a concern. Generally, the thicker the coating, the better the coating performance, i.e. in terms of corrosion resistance, durability and protection of the surface. In order to obtain a thicker coating, however, the flame spray must be directed to the surface for a longer period of time to allow more material to be deposited. In turn, the longer the exposure of the surface to the flame spray, the higher the temperature of the coating which is reached during its deposition. If the temperature is too high, then the desirable properties of the polymer can be adversely affected by polymer degradation, and in severe cases burning or scorching may occur. Conversely, the higher the upper temperature on the coating before properties are adversely affected, the thicker the coating which can be achieved in one application. When the plastic cannot be laid down thick enough in an initial coating, subsequent passes may be required to lay the plastic down in a number of layers. This has the inherent disadvantages of requiring additional labor and creating stresses in the coating between the various layers of the plastic which can lead to undesirable defects in the overall plastic coating.
Additionally, the properties of the plastic coating are a major concern. Desirable properties include thickness and adhesion, as previously mentioned, and also other mechanical and surface properties such as smoothness, gloss, impact strength and the avoidance of pinholes. Accordingly, the selection of coating materials and application techniques is dictated by the desired properties of the resulting coating. It is also desirable to facilitate the coating application process. The application rate is of economic importance, of course, in order to minimize the time and labor that it takes to form the coating on the surface.
The ease of application is also important from the standpoint that the process variables should allow for a wide margin of error or "forgiveness" in their selection. This would have the direct result that the flame spray coating can be applied in a wider variety of situations and environmental extremes without operator difficulty.
In conventional thermal spray systems for plastics, the applicators must contend with relatively narrow temperature "application windows" or "envelopes." For example, on the low thermal input end, i.e., not enough heating of the polymer, substrate adhesion can be low or marginal due to insufficient polymer melting and/or substrate wetting, and pinholing can occur due to poor particle-to-particle coalescence on the substrate surface. On the other hand, if the plastic is overheated in the flame, excessive crosslinking can result in higher melt viscosity, poor melt flow, reduced cohesive strength, low adhesion, discoloration and scorching, and pinholing can also result from out-gassing of degradation by-products and/or poor particle-to-particle coalescence arising from high melt viscosity.
The effect of the particle size of the plastic flame-sprayed from the gun has also been noted. For example, Japanese Patent Publication No. 62-2866 (1979) describes a flame spraying operation using a modified polyethylene containing 0.01 to 10 parts by weight, per 100 parts by weight of the polyethylene, of an unsaturated carboxylic acid or anhydride, having a melt tension from 0.5 to 15 g, and a particle size distribution from 30 to 200 mesh. It was reported that particle diameters smaller than 200 mesh result in the formation of air bubble voids in the coating, but that particle diameters exceeding (larger than) 30 mesh lead to nonuniform coatings which are not smooth and have an inferior "orange peel" appearance.
Various methods of obtaining small particles of polyolefins for coatings have been used. For example, U.S. Pat. No. 3,932,368 to McConnell describes the cryogenic grinding of carboxylated polyolefins to less than about 20 mesh size for coating substrates using a fluidized wet coating process, and to less than about 100 mesh size for electrostatic spray coating. This patent also describes the use of thermal, oxidative and ultraviolet radiation stabilizers in the powdered polyolefin.
The flame spray coating technique has been used with polyethylenes containing other additives, and with chlorinated polyethylenes. This is illustrated by U.S. Pat. No. 2,962,387 to Noeske, which describes the flame spray application of chlorinated polyethylenes with a critical chlorine content to minimize shrinkage of the coating following its application, and by U.S. Pat. No. 2,676,932 to Deniston, which describes a flame spraying composition containing polyethylene and a diethylene glycol stearate wax.